Magnetic element using isolated domains in rare earth orthoferrites

ABSTRACT

The disclosure describes a structure useful for isolated magnetic domain devices. The structure is a composite of samarium-rich orthoferrite and samarium-poor orthoferrite one of which is epitaxially deposited on the other. The result is that the epitaxial layer is magnetically isolated from the substrate since in these materials the crystallographic direction of net magnetic moment is 90* apart.

[ Mar. 7, 1972 MAGNETIC ELEMENT USING ISOLATED DOMAINS IN RARE EARTH ORTHOFERRITES Inventor: Raymond Wolfe, New Providence, NJ.

Assignee: Bell Telephone Laboratories, Incorporated,

Murray Hill, NJ.

Filed: Feb. 5, 1968 Appl. No.: 703,162

References Cited OTHER PUBLICATIONS Bell System Technical Journal, Vol. XLVl, No. 8, pp. 1,901- 1,925

Primary Examiner-William D. Martin Assistant Examiner-Bernard D. Pianalto AttameyR. J. Guenther and Edwin B. Cave [57] ABSTRACT The disclosure describes a structure useful for isolated magnetic domain devices. The structure is a composite of samari- [1.8. CI ..ll7/234, 117/235, 252/6257 urn-rich rthoferrite and ama 'ium po()f onhofen-ile one of Int. Cl. ..H0lf 10/02, l-l0lf 10/04 which is epitaxially deposited on the other. The result is that Field of Search ..1 17/234, 235, 239; 252/6257 the epitaxial layer is magnetically isolated from the Substrate since in these materials the crystallographic direction of net magnetic moment is 90 apart.

9 Claims, Drawing Figures (/2 r I I I I I l MAGNETIC ELEMENT USING ISOLATED DOMAINS IN RARE EARTH ORTIIOFERRITES This invention relates to memory and logic devices based upon isolated magnetic domains in orthoferrites.

A new class of memory and logic devices has recently been proposed in which a relatively thin sheet of rare earth orthoferrite is used to store and process information contained in the sheet as small isolated magnetic domains. The properties and applications for such devices are described in The Bell System Technical Journal, Vol. XLVI, No. 8, pp. l,90l1,925 and in U.S. Pat. applications Ser. Nos.- 579,931; 579,904; 579,905; 579,866; filed Sept. 16, 1966, U.S. Pat. Nos. 3,460,116, 3,393,769, 3,470,547, and 3,471,840, respectively; Ser. No. 609,84l filed Jan. 17, 1967 and Ser. No. 644,35l filed June 7, I967, filed by A. H. Bobeck et al., and U.S. Pat. application Ser. No. 686,276 filed Nov. 28, 1967 by R. C. Sherwood and A. A. Thiele, and U.S. Pat. application Ser. No. 629,993 filed Apr. ll, 1967 by R. C. LeCraw et al., U.S. Pat. Nos. 3,505,660, 3,506,975, 3,534,341, and 3,506,974, respectively. The basic requirement of the material from which these devices are made is that it have a high uniaxial magnetic anisotropy in the direction normal to the storage plane with a relatively weak moment. This permits the formation of a stable isolated domain having a fairly small wall coercivity. The small wall coercivity makes it possible to move the isolated domains with relatively small fields once they are formed so that the information represented by arrays of isolated domains can be shifted and interacted to process the information. The required magnetic properties are exhibited by yttrium orthoferrite and all the rare earth orthoferrites, specifically:

where M is yttrium or a rare earth element having an atomic number from 57 to 71. These materials have been investigated and the magnetic properties of interest for the present application are known in the literature. Most of the devices produced heretofore employed rare earth orthoferrite crystals produced by flux growth as described in Journal of American Chemical Society, Vol. 78, page 4,259 (1956). A more recent method for preparing these materials by a hydrothermal technique is described in U.S. Pat. application Ser. No. 660,643 filed Aug. 15, I967, U.S. Pat. No. 3,485,759, by E. D. Kolb, R. A. Laudise, E. G. Spencer and D. L. Wood.

According to the present invention a new orthoferrite structure is described especially suited for the devices referred to above. This structure is a composite of two rare earth or yttrium orthoferrite layers in which the layers have different magnetic characteristics designed to enhance the magnetic performance of the device. In its general form this structure is composed of a layer relatively rich in samarium orthoferrite in association with a layer relatively poor in samarium orthoferrite. Both layers are single crystals, one being formed epitaxially on the other.

Samarium orthoferrite is unique among the rare earth orthoferrites in that its spin flop temperature is above room temperature. In practical terms this means that its net moment at room temperature lies parallel to the a-axis while the other rare earth orthoferrites and yttrium orthoferrite have a net moment parallel to the c-direction. Thus the composite structure has a layer with a moment normal to its major plane and a layer having a moment parallel to the major plane. As a consequence of this arrangement isolated domains of a desirable size can be stored and processed in the active layer, i.e., the layer having a magnetization direction normal to the major plane of the device. In addition, readout of the domains is facilitated without interference from the inactive or substrate layer.

These and other aspects of the invention will be more fully explained in the following detailed description. In the drawing:

FIG. 1 is a perspective view of the general composite orthoferrite structure forming the basis for this invention; and

FIG. 2 is a schematic representation of an arrangement for processing magnetic domains using the structure of FIG. 1.

In FIG. 1 the substrate or support layer 10 is a single crystal of samarium or samarium-rich orthoferrite having its net magnetic moment in the a-direction as indicated. The dimensions of this layer are unimportant. The layer 11 is a thin film of a rare earth or yttrium orthoferrite having its direction of mag' netization normal to the major plane of the film which, in this case, is the c-axis. The layer 11 is also a single crystal, deposited epitaxially by known methods, on the matched substrate 10. The thickness of the layer 11 should be in the range of 0.1 mil to 5 mils to obtain a domain having a useful size. An

' isolated domain is indicated schematically'at 12. The advantage of the unique combination of samarium-rich and and samarium-poor layers is that it is only with this combination that the substrate and the epitaxial layer can have the same crystal orientation but have different directions of magnetization.

A similar structure can be constructed by reversing the layers 10 and 11, and changing the crystal orientation so that the a-axis is normal to the major plane of the structure and the c-axis is in the plane. The active layer 11 must now be samarium-rich so that the net moment is in the a-direction and the substrate 10 is a samarium-poor orthoferrite so that its net moment is in the c-direction.

FIG. 2 is a schematic representation showing the elements of a complete device. The figure shows a single wall domain propagation device 20 including a structure 21 of magnetic material in which single wall domains, represented by a domain D, are moved from input to output positions. The structure 21 is formed of the composite orthoferrite structure of this invention. It is assumed to be saturated magnetically in a negative direction away from the viewer in FIG. 2. A single wall domain D, then, may be represented by an encircled plus sign where the plus sign indicates the direction of magnetization and the circle represents the single domain wall. Similarly, a source of positive magnetization (domains) is represented at 22 by an encompassed plus sign.

A conductor 23 couples an input position which is adjacent source 22. Conductor 23 is connected between an input pulse source 24 and ground. When source 24 pulses conductor 23, the latter generates a field which attracts a domain from source 22. If a pulse is absent, at a particular time, from conductor 23, an absence of a domain occurs at the input position. The presence and absence of domains represents binary ones and zeros, respectively, for propagation to a remote output position. i

The movement of domains in a domain propagation medium is controlled by patterns of propagation fields which attract domains. Conductors X1, X2, and X3 are shown in FIG. 2 for this purpose. The conductors couple consecutively offset positions along the path of propagation between input and output positions, and the pattern of coupling repeats. Thus, conductor X1 couples first, fourth, etc., positions along the path of propagation as shown in the figure. Similarly, conductors X2 and X3 couple the second, fifth, etc., and couple the third, sixth, etc., positions between the input and output positions respectively. Conductors X1, X2, and X3 are connected to an X driver 25 and are pulsed consecutively thereby during operation for generating consecutively offset fields for advancing domains. The repeat pattern of couplings causes those fields to be generated in patterns which synchronize the advance of next consecutive bits thus providing a built-in timing with which inputs and outputs are synchronized also. Information is advanced in this manner until an output position is reached.

The output position is designated by a broken circle 26. An arrow 27 represents an output coupling to the output position. Such a coupling may be electrical or optical in nature. If electrical, arrow 27 represents a conductor connected between utilization circuit 28 and ground. If optical, arrow 27 may represent a beam of polarized monochromatic light and utilization circuit 28 would be a photocell and analyzer combination arranged to detect the presence and absence of polarized light via Faraday rotation for indicating the presence and absence of a domain in the output position respectively.

'lolnn MM Only a single domain propagation channel is shown in FIG. 2. Single wall domains, however, are capable of multidimensional movement. Thus, propagation channels in the Y- direction orthogonal to the channel shown may be present. Moreover, channels parallel to the one shown may also be present and information may be moved controllably from channel to channel. To move information in a Y-direction, Y- propagation conductors are used. These are operated in a three-phase manner as described for the X-conductors and are coupled to positions along the Y-direction arranged similarly to those described for the X-couplings. FIG. 2 shows the Y- conductors Y1, Y2 and Y3 connected to a Y-driver 30.

A magnet, represented by the block M in FIG. 2, suitably shaped to provide a field of the requisite magnitude, is positioned adjacent to the orthoferrite structure 21. This magnet serves the function of biasing the magnetic structure so that essentially circular domains can be formed.

Source 24, drivers 25 and 30 and circuit 28 are connected to a control circuit 31 via conductors 32, 33, 34 and 35. The various sources, drivers, and circuits may be any such elements capable of operating in accordance with this invention.

The readout schemes described above require interrogation by either a light beam or an electric signal. Referring back to FIG. I, if the'substrate is magnetized in a direction normal to the principal plane of the device then the layer 10 will adversely affect either kind of interrogating signal. This is because the domains would be aligned in a direction which would produce effective Faraday rotation of light or would dampen the electrical signal produced by the lateral motion of the isolated magnetic domain. However, since the domains are, in accordance with the invention, aligned in the major plane of the device they will exert no effect on the readout signal.

Mixed crystals of rare earth and yttrium orthoferrites are known and may, in some cases, be especially useful for adj usting the wall energy of the domains to an optimum value for the particular temperature (normally room temperature) at which the device is to operate. Mixed crystals of all the rare earth orthoferrites can be produced according to known procedures. However, for the purpose of this invention a mixed samarium-rare-earth or samarium'yttrium orthoferrite is of interest. The preparation of such a material, R Sm FeO where R is any rare earth or yttrium, is described in application Ser. No. 660,643 referred to above. Properties of these materials are discussed in Physics Letters, Vol. 25A, No. 4, page 297, (1967). The specific property of interest in conneetion with the devices discussed herein is the reorientation temperature, that is, the temperature at which the magnetic easy direction changes from the c-axis to the a-axis. The reorientation temperature for samarium orthoferrite, Sm- FeO occurs typically through the temperature range of 468 to 487 K. This reorientation temperature range can be lowered toward room temperature by the addition of other orthoferrites, all of which, as indicated previously, have reorientation temperature ranges considerably below room temperature. In a few cases the reorientation temperature, if it exists at all, occurs at such a low temperature that it has not been detected. Certain advantages can accrue from such an adjustment in terms of the magnetic properties of the orthoferrite. The anisotropy can be reduced if desired by such a substitution. The amount of rare earth or yttrium required to reduce the reorientation temperature range to below room temperature varies depending upon the element being added. Therefore, the composition suitable for the layer 10 of FIG. 1, or the layer 11 of the alternative structure described previously, cannot be defined generally in terms of its composition. It is perhaps more meaningful to specify that this layer be composed of an orthoferrite containing sufiicient samarium (usually more than 50 percent) so that its reorientation temperature range occurs above room temperature. In terms of the function of the structure the composition of this layer is defined as an orthoferrite having a net magnetic moment in a direction parallel to the plane of the structure or, in the alternative embodiment, normal to the plane of the structure. The

layer'll of FIG. 1, or the layer 10 in the alternative structure, which is referred to above as the samarium-poor layer can of course be devoid of Samarium. The appropriate prescription for this layer is that it be composed of an orthoferrite in which the net magnetic moment at the operating temperature of the device is orthogonal to the net magnetic moment in the associated orthoferrite layer 10.

The orthoferrites have been generally defined by the formula MFeO However, the term orthoferrite" actually encompasses compounds in which the iron is partially substituted up to 50 percent atomic percent by some suitable element such as chromium or aluminum. Such a compound will have properties useful for this invention.

The term epitaxial or epitaxially formed used herein is intended to characterize a thin film having an essentially identical crystal structure and orientation as the substrate associated with it.

Exemplary combinations of compositions useful for this invention are the following:

In each case the crystallographic c-axis is normal to the major plane of the structure. If the film is epitaxially grown on the a-face of the substrate then the compositions of the layers can be merely reversed to achieve the same result.

Various additional modifications and extensions of this invention will become apparent to those skilled in the art. All such variations and deviations which basically rely on the teachings through which this invention has advanced the art are properly considered within the spirit and scope of this invention.

What is claimed is:

l. A novel structure for use in a magnetic memory or logic device comprising a substrate composed of a single-crystal magnetic orthoferrite and an epitaxial, single-crystal, magnetic orthoferrite film having a thickness in the range of 0.1 mil to 5 mils overlying the substrate and having a composition different from that of the substrate such that its net magnetic moment is in a direction normal to the plane of the substrate and to the direction of net magnetization of the substrate.

2.'A novel structure for use in a magnetic memory or logic device comprising a substrate composed of a samarium-rich orthoferrite in which the net moment of magnetization at the operating temperature of the device is in the crystallographic a-direction and the plane of the substrate is normal to the crystallographic c-direction, and an epitaxial, single crystal, orthoferrite film having a thickness in the range of 0.1 mil to 5 mils and overlying said substrate the orthoferrite film having a composition such that its net moment of magnetization is in the crystallographic c-direction.

3. The structure of claim 2 wherein the substrate is SmFeO 4. The structure of claim 3 wherein the composition of the film is selected from the group consisting of yttrium orthoferrite, a rare earth orthoferrite, a mixture thereof, and a mixture of rare earth orthoferrites.

5. The structure of claim 2 wherein the substrate has the composition where R is yttrium or a rare earth and x is a value such that the reorientation temperature for the magnetization of the composition has a temperature range above room temperature.

6. A novel structure for use in a magnetic memory or logic device comprising a substrate composed of a single-crystal orthoferrite in which the net moment of magnetization at the operating temperature of the device is in the crystallographic c-direction and the plane of the substrate is normal to the crystallographic a-direction, and an epitaxial single crystal samarium-rich orthoferrite film of a thickness in the range of 0.1 mil to 5 mils overlying said substrate the samarium-rich orthoferrite film having a composition such that its net moment of magnetization is in the crystal lographic a-direction.

7. The structure of claim 6 wherein the samarium-rich orthoferrite film is SmFeO 8. The structure of claim 7 wherein the composition of the substrate is selected from the group consisting of yttrium 10 

2. A novel structure for use in a magnetic memory or logic device comprising a substrate composed of a samarium-rich orthoferrite in which the net moment of magnetization at the operating temperature of the device is in the crystallographic a-direction and the plane of the substrate is normal to the crystallographic c-direction, and an epitaxial, single crystal, orthoferrite film having a thickness in the range of 0.1 mil to 5 mils and overlying said substrate the orthoferrite film having a composition such that its net moment of magnetization is in the crystallographic c-direction.
 3. The structure of claim 2 wherein the substrate is SmFeO3.
 4. The structure of claim 3 wherein the composition of the film is selected from the group consisting of yttrium orthoferrite, a rare earth orthoferrite, a mixture thereof, and a mixture of rare earth orthoferrites.
 5. The structure of claim 2 wherein the substrate has the composition RxSm1 xFeO3 where R is yttrium or a rare earth and x is a value such that the reorientation temperature for the magnetization of the composition has a temperature range above room temperature.
 6. A novel structure for use in a magnetic memory or logic device comprising a substrate composed of a single-crystal orthoferrite in which the net moment of magnetization at the operating temperature of the device is in the crystallographic c-direction and the plane of the substrate is normal to the crystallographic a-direction, and an epitaxial single crystal samarium-rich orthoferrite film of a thickness in the range of 0.1 mil to 5 mils overlying said substrate the samarium-rich orthoferrite film having a composition such that its net moment of magnetization is in the crystal lographic a-direction.
 7. The structure of claim 6 wherein the samarium-rich orthoferrite film is SmFeO3.
 8. The structure of claim 7 wherein the composition of the substrate is selected from the group consisting of yttrium orthoferrite, a rare earth orthoferrite, a mixture thereof, and a mixture of rare earth orthoferrites.
 9. The structure of claim 6 wherein the samarium-rich orthoferrite film has the composition RxSm1 xFeO3 where R is yttrium or a rare earth and x is a value such that the reorientation temperature for the magnetization of the composition has a temperature range above room temperature. 